to examine the effect of the time increment on the results carefully because the program does not
enforce any creep ratio control by default.You can always enforce a creep limit ratio using the creep
ra tio control option in commands CRPLIM or CUTCONTROL,CRPLIMIT. A recommended value for a
creep limit ratio ranges from 1 to 10.The ratio may vary with materials so your decision on the best
value to use should be based on your own experimentation to gain the required performance and ac-
curacy. For larger analyses, a suggestion is to first perform a time increment convergence analysis on
a simple small size test.
Tools are available to help you determine the coefficients for all of the implicit creep options defined
in TB,CREEP. The TBFT command allows you to compare your experimental data with existing material
data curves and visually fit your curve for use in the TB command. All of the TBFT command capability
(except for plotting) is available via batch and interactive (GUI) mode. See Material Curve Fitting for
more information.
8.4.1.6.2. Explicit Creep Procedure
The basic procedure for using the explicit creep method involves issuing the TB command with Lab
= CREEP and choosing a creep equation by adding the appropriate constant as an argument with the
TBDATA command.TBOPT is either left blank or = 0.The following example input uses the explicit
creep method. Note that all constants are included as arguments with the TBDATA command, and that
there is no temperature dependency.
TB,CREEP,1
TBDATA,1,C1,C2,C3,C4, ,C6For the explicit creep method, you can incorporate other creep expressions into the program by using
User Programmable Features (see the Guide to User-Programmable Features).
For highly nonlinear creep strain vs. time curves, a small time step must be used with the explicit creep
method. Creep strains are not computed if the time step is less than 1.0e-6. A creep time step optimiz-
ation procedure is available (AUTOTS and CRPLIM) for automatically adjusting the time step as appro-
priate.
8.4.1.7. Shape Memory Alloy Material Model
The Shape Memory Alloy (TB,SMA) material behavior option describes the superelastic behavior of
nitinol alloy. Nitinol is a flexible metal alloy that can undergo very large deformations in loading-unloading
cycles without permanent deformation. The material behavior has three distinct phases: an austenite
phase (linear elastic), a martensite phase (also linear elastic), and the transition phase between these
two.
For more information, see Shape Memory Alloy (SMA) in the Material Reference.
8.4.1.8. Viscoplasticity
Viscoplasticity is a time-dependent plasticity phenomenon, where the development of the plastic strain
is dependent on the rat e of loading. The primary application is high-temperature metal-forming (such
as rolling and deep drawing) which involves large plastic strains and displacements with small elastic
strains. (See Figure 8.12:Viscoplastic Behavior in a Rolling Operation (p. 214).)
Viscoplasticity is defined by unifying plasticity and creep via a set of flow and evolutionary equations.
A constraint equation preserves volume in the plastic region.
For more information about modeling viscoplasticity, see Nonlinear Stress-Strain Materials.
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